The present invention relates generally to joint mechanisms, and more specifically to a new class of large displacement spherical joints. The spherical joint has long been a standard of mechanical design. The general nature of prior art spherical joints is shown in FIG. 1. Here a spherical body 10 makes bearing contact with a bearing cup 12 on a bearing surface 11. The term `bearing contact` is intended to describe a juxtaposition between two mechanical components which allows them to easily move relative to each other restricted only by the structure of the bearing. The juxtaposition can be surfaces sliding on each other, with or without lubrication, or can be mediated by any of a wide range of conventional bearing elements, such as balls, rollers, and the like. The term `bearing surface` shall be intended to include the mechanisms which comprise such mediated bearing contacts.
Such bearings generally perform best if bearing surface 11 is spherical in shape, and has a radius substantially equal to that of the spherical body 10, but neither condition is strictly necessary. For example, bearing cup 12 can take the form of a hollow tube, with the bearing surface taking the form of a ring on which the spherical body and the bearing cup make bearing contact. In another example, bearing cup 12 can be replaced by three properly spaced and oriented flat bearing pads, and the resulting bearing surface will have equivalent functionality. Of course, such a bearing surface would be expected to wear at a much faster rate than the illustrated structure.
A first shaft 13 is affixed to spherical body 10, generally but not necessarily aligned along a radius of the spherical body 10. A second shaft 14 can be affixed to bearing cup 12, although other mounting techniques, such as attaching bearing cup 12 to a joint base, can be used. Finally, spherical retainer 15 provides a second bearing surface 16 for spherical body 10. The two bearing surfaces are positioned so that spherical body 10 cannot move, other than through rotation in place, relative to the bearing cup and the spherical retainer.
The structure described above allows considerable freedom of motion of the two shafts relative to each other. Using the orientation of the second shaft as a reference, the first shaft can move freely within a full cone angle a while at the same time rotating freely about its own axis.
The primary restriction on the amount of movement allowed by a spherical joint is the interference between the first shaft and the spherical retainer when an attempt is made to move the first shaft to a position outside the allowed full cone of motion. This interference results from the need to provide a mechanical retainer to keep the spherical body in contact with the bearing cup so that the rotary motion thereof is well-defined.
In most commercially available mechanically restrained spherical joints the available full cone angle .alpha. is less than 40 degrees, and examples are simply not available with .alpha.&gt;60 degrees. As prior art spherical joints were primarily used to accommodate minor shaft misalignments, the limited full cone angle was not a serious limitation.
There is a variety of prior art spherical joint that allows access to larger full cone angles. In these joints, the spherical retainer does not appear, and the spherical body is held within the bearing cup by magnetic attraction. Such joints, however, cannot tolerate large tensile forces, and are susceptible to dislocation under small dynamic or static forces which do not directly press the spherical body into the bearing cup. Such magnetic spherical joints thus have very limited fields of usage.
New applications for spherical joints have recently arisen for which a large allowed full cone angle is a great advantage. These include many of the parallel mechanisms on which flexible machining platforms and robotic manipulators are now based. In such applications, the greater allowed full cone angle contributes to increasing the workspace of the machine. The result is dramatic increase in efficiency, in large part driven by reducing the total setup time as a workpiece is dismounted and reoriented.
There is a prior art spherical joint that has the potential for providing somewhat larger allowed deflection angles, perhaps to full cone angles as large as 120-140 degrees, although to Applicants knowledge such have not been commercially available. This is the joint described in U.S. Pat. No. 4,628,765, Dien and Luce, issued Dec. 16, 1986 (now expired). In this patent is disclosed a spherical joint comprising a spherical body 20 mounted within a ring-shaped bearing 21 which allows rotation in any direction (see FIG. 2). A pair of semi-circular yokes 22 and 23 oriented along perpendicular axes provide a means to characterize and control the position of a shaft 24 attached to the spherical body. The ring-shaped bearing 21 mechanically retains the spherical body 20 by enclosing a diameter of the spherical body. This, however, has the effect of limiting the allowed deflection angle to values substantially less than 90 degrees. Such a ring-shaped bearing is also difficult and expensive to incorporate into a commercial joint.
There are numerous ways in which a concatenation of revolute joints can be assembled to mimic the behavior of a spherical joint. An example is shown in FIG. 3, where a `spherical` joint between a first shaft 30 and a second shaft 31 is implemented by combining the effect of a revolute joint 32 imbedded in the end of second shaft 31 with the effect of a revolute fork joint 33 mounted upon revolute joint 32 so that the axis of revolution of the two joints are perpendicular. First shaft 30 is mounted on revolute fork joint 33 via revolute joint 34 so that first shaft 30 is free to turn about its own axis. In this design, three pairs of axles and matching bearings, together with a collection of precision machined and assembled components, are required to mimic the behavior of a spherical joint. In addition, the joint stability which follows naturally from having a spherical body firmly set on an appropriate bearing surface can only be achieved here by insisting on extremely tight manufacturing tolerances. Maintenance, useful life, and other practical considerations fall solidly on the side of the true spherical joint. In the end, even though the joint illustrated in FIG. 3 mimics the behavior of a spherical joint capable of very large deflection angles, in most cases it is not a practical option.
There is a need for a precision spherical joint which is mechanically stable and capable of large (i.e., .alpha.&gt;60 degrees) full cone angles while remaining resistant to mechanical joint dislocation. Applicants have addressed this need by developing a new type of spherical joint which satisfies these criteria and more.